1. Field of the Invention
The invention relates to a new composite material and, more particularly, to a copper composite material of low thermal expansion and high thermal conductivity, and various kinds of uses such as a semiconductor equipment in which this composite material is used.
2. Description of the Prior Art
Techniques related to the conversion and control of electric power and energy by means of electronic devices and, in particular, power electronic devices used in an on-off mode and power conversion systems as applied techniques of these power electronic devices are called power electronics.
Power semiconductor devices with various kinds of on-off functions are used for power conversion. As such semiconductor devices, there are put to practical use not only rectifier diodes which contain pn junctions and which have conductivity only in one direction, but also thyristors, bipolar transistors, MOS FETs (metal oxide semiconductor field effect transistors) and etc. which differ from each other in various combinations of pn junctions. Moreover, there are also developed insulated gate type bipolar transistors (IGBTs) and gate turn-off thyristors (GTOs) which have a turn-off function by gate signals.
These power semiconductor devices causes the generation of heat by energization and the amount of generated heat tends to increase because of the high capacity design and high speed design of power semiconductor devices. In order to prevent the deterioration of the properties of a semiconductor device and the shortening of its service life from being caused by the heat generation, it is necessary to provide a heat-radiating portion to thereby suppress a temperature rise in and near the semiconductor device. Because copper has a high thermal conductivity of 393 W/mxc2x7k and is inexpensive, this metal is generally used as heat-radiating members. However, because a heat-radiating member of a semiconductor equipment provided with a power semiconductor device is bonded to Si having a thermal expansion coefficient of 4.2xc3x9710xe2x88x926/xc2x0 C., a heat-radiating member having a thermal expansion coefficient close to this value is desired. Because the thermal expansion coefficient of copper is as large as 17xc3x9710xe2x88x926/xc2x0 C., the solderability of copper to the semiconductor device is not good. Therefore, materials with a coefficient of thermal expansion close to that of Si, such as Mo and W, are used as a heat-radiating member or installed between the semiconductor device and the heat-radiating member.
On the other hand, integrated circuits (ICs) formed by integrating electronic circuits on one semiconductor chip are sorted according to their functions into a memory, logic, microprocessor, etc. They are called electronic semiconductor devices in contrast with power semiconductor devices. The integration degree and operating speed of these semiconductor devices have been increasing year by year, and the amount of generated heat has also been increasing accordingly. On the other hand, an electronic semiconductor device is generally housed in a package in order to prevent troubles and deterioration by shutting it off from the surrounding atmosphere. Most of such packages are either a ceramic package or a plastic package, in which ceramic package a semiconductor device is die-bonded to a ceramic substrate and sealed, and in which plastic package a semiconductor device is encapsulated with resins. In order to meet requirements for higher reliability and higher speeds, a multi-chip module (MCM) in which multiple semiconductor devices are mounted on one substrate is also manufactured.
In a plastic package, a lead frame and terminals of a semiconductor device are connected by means of a bonding wire and encapsulated with plastics. In recent years, with an increase in the amount of generated heat of semiconductor devices, a package in which the lead frame has a heat-dissipating property and another package in which a heat-radiating board for heat dissipation is mounted have also come to be thought of. Although copper-base lead frames and heat-radiating boards of large thermal conductivity are frequently used for heat dissipation, there is such a fear as problems may occur due to a difference in thermal expansion from Si.
On the other hand, in a ceramic package, a semiconductor device is mounted on a ceramic substrate on which wiring portions are printed, and the semiconductor device is sealed with a metal or ceramic cap. Moreover, a composite material of Cuxe2x80x94Mo or Cuxe2x80x94W or a kovar alloy is bonded to the ceramic substrate and used as a heat-radiating board, and in each of these materials there is required an improvement in workability and a low cost as well as lower thermal expansion design and higher thermal conductivity design.
In an MCM (multi-chip module), multiple semiconductor devices are mounted as bare chips on the thin-film wiring formed on an Si or a metal or a ceramic substrate, are housed in a ceramic package, and are encapsulated with a lid. When the heat-radiating property is required, a heat-radiating board and a heat-radiating fin are installed in the package. Copper and aluminum are used as the material for metal substrates. Although copper and aluminum have the advantage of a high thermal conductivity, these metals have a large coefficient of thermal expansion and have inferior compatibility with semiconductor devices. For this reason, Si and aluminum nitride (AlN) are used as the substrate of a high-reliability MCM. Further, because the heat-radiating board is bonded to the ceramic package, a material having good compatibility with the package material in terms of coefficient of thermal expansion and having a large thermal conductivity is desired.
As mentioned above, all semiconductor equipments each provided with a semiconductor device generate heat during operation, and the function of the semiconductor device may be impaired if the heat is accumulated. For this reason, a heat-radiating board with excellent thermal conductivity for dissipating the heat to the outside is necessary. Because a heat-radiating board is bonded directly or via an insulating layer to the semiconductor device, its compatibility with the semiconductor device is required not only in thermal conductivity, but also in thermal expansion.
The materials for semiconductor devices presently in use are mainly Si (silicon) and GaAs (gallium arsenide). The coefficients of thermal expansion of these two materials are 2.6xc3x9710xe2x88x926/xc2x0 C. to 3.6xc3x9710xe2x88x926/xc2x0 C. and 5.7xc3x9710xe2x88x926/xc2x0 C. to 6.9xc3x9710xe2x88x926/xc2x0 C., respectively. As the materials for heat-radiating boards having a coefficient of thermal expansion close to these values, AlN, SiC, Mo, W, Cuxe2x80x94W, etc., have been known. However, because each of them is a single material, it is difficult to control to an arbitrary level the coefficients of heat transfer and thermal conductivity and, at the same time, there is a problem that they are poor in workability and require a high cost.
Recently, Alxe2x80x94SiC has been proposed as a material for heat-radiating boards. This is a composite material of Al and SiC and the coefficients of heat transfer and thermal conductivity can be controlled in a wide range by changing the proportions of the two components. However, this material has the disadvantage of very inferior workability and a high cost. A Cuxe2x80x94Mo sintered alloy is proposed in JP-A-8-78578, a Cuxe2x80x94Wxe2x80x94Ni sintered alloy being proposed in JP-A-9-181220, a Cuxe2x80x94SiC sintered alloy being proposed in JP-A-9-209058, and an Alxe2x80x94SiC is proposed in JP-A-9-15773. In these publicly known composite materials obtained by powder-metallurgical processes, the coefficient of thermal expansion and thermal conductivity can be controlled in wide ranges by changing the ratio of the two components. However, their strength and plastic workability are low and the manufacture of sheets is difficult. In addition, there are problems of a high cost related to the production of powder, an increase in the steps of manufacturing process and etc.
The object of the invention is to provide a composite material excellent in plastic workability, a method of manufacturing the composite material, a semiconductor equipment in which the composite material is used, a heat-radiating board of the semiconductor equipment, an electrostatic adsorption device, and a dielectric board of the electrostatic adsorption device.
As a result of a repetition of various researches, the present inventors have found that the above problems can be solved by a composite material composited through the steps of melting Cu of high thermal conductivity and Cu2O of lower thermal expansion than Cu and dispersing each of these materials.
According to the first aspect of the invention, there is provided a composite material comprising a metal and an inorganic compound preferably having a smaller coefficient of thermal expansion than the metal, most of the compound being granular grains with a grain size of preferably not more than 50 xcexcm and dendrites.
According to the second aspect of the invention, the compound comprises dendrites each having a bar-like stem and branches of a granular shape.
According to the third aspect of the invention, there is provided a composite material comprising a metal and an inorganic compound, most of the compound are granular grains with a grain size of 5 to 50 xcexcm and dendrites, and 1 to 10% of the whole compound are fine grains with a grain size of not more than 1 xcexcm.
According to the fourth aspect of the invention, there is provided a composite material comprising a metal and an inorganic compound, the coefficient of thermal expansion or thermal conductivity being larger in a solidification direction than in a direction vertical to the solidification direction.
Most preferably, the composite material of the invention may be one comprising copper and copper oxide.
According to the fifth aspect of the invention, there is provided a composite material comprising a metal and an inorganic compound having a shape of bar with a diameter of 5 to 30 xcexcm, and preferably, not less than 90% of the whole of the inorganic compound in terms of the area percentage of section is in the shape of a bar with a diameter of 5 to 30 xcexcm.
The composite material of the invention may comprise copper and copper oxide and may be plastically worked.
According to the sixth aspect of the invention, there is provided a composite material comprising copper, copper oxide and incidental impurities, the content of the copper oxide being 10 to 55% by volume, copper oxide being made to be dendrites, the coefficient of linear expansion in a temperature range from room temperature to 300xc2x0 C. being 5xc3x9710xe2x88x926/xc2x0 C. to 17xc3x9710xe2x88x926/xc2x0 C., and the thermal conductivity thereof at room temperature is 100 to 380 W/mxc2x7k. This composite material has anisotropy.
According to the seventh aspect of the invention, there is provided a composite material comprising copper, copper oxide, preferably cuprous oxide (Cu2O) and incidental impurities, the content of copper oxide being preferably 10 to 55% by volume, the copper oxide being provided with a shape of bars each oriented in one direction, the coefficient of linear expansion of the copper oxide in a temperature range from room temperature to 300xc2x0 C. is 5xc3x9710xe2x88x926/xc2x0 C. to 17xc3x9710xe2x88x926/xc2x0 C., and the thermal conductivity thereof at room temperature is 100 to 380 W/mxc2x7k. In this composite material, the thermal conductivity in the oriented direction is higher than that in the direction at right angles to the oriented direction, and the difference between the two is preferably 5 to 100 W/mxc2x7k.
According to the eighth aspect of the invention, there are provided a manufacturing method in which a metal and an inorganic compound forming a eutectic structure with this metal are melted and solidified and, in particular, a manufacturing method of a composite material comprising copper and copper oxide. This manufacturing method preferably comprises the step of preparing a raw material of copper or copper and copper oxide, melting the raw material in an atmosphere having a partial pressure of oxygen of 10xe2x88x922 Pa to 103 Pa followed by casting, performing heat treatment thereof at 800xc2x0 C. to 1050xc2x0 C., and preferably performing cold or hot plastic working thereof.
According to the ninth aspect of the invention, there is provided a heat-radiating board for semiconductor equipment, which board is made of the above composite material. In the heat-radiating board for the semiconductor equipment may have a nickel plating layer on its surface.
According to the tenth aspect of the invention, there is provided a semiconductor equipment comprising an insulating substrate mounted on a heat-radiating board, and a semiconductor device mounted on the insulating substrate, said heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the eleventh aspect of the invention, there is provided a semiconductor equipment comprising a semiconductor device mounted on a heat-radiating board, a lead frame bonded to the heat-radiating board, and a metal wire for electrically connecting the lead frame to the semiconductor device, the semiconductor device being resin-encapsulated, and the heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the twelfth aspect of the invention, there is provided a semiconductor equipment which comprises a semiconductor device mounted on a heat-radiating board, a lead frame bonded to the heat-radiating board, and a metal wire for electrically connecting the lead frame to the semiconductor device, the semiconductor device being resin-encapsulated, at least the face of the heat-radiating board which face is opposed to the connection face of the semiconductor device is opened, and the heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the thirteenth aspect of the invention, there is provided a semiconductor equipment comprising a semiconductor device mounted on a heat-radiating board, a ceramic multilayer substrate provided with a pin for connecting external wiring and an open space for housing the semiconductor device in the middle thereof, and a metal wire for electrically connecting the semiconductor device to a terminal of the substrate, and both of the heat-radiating board and the substrate being bonded to each other so that the semiconductor device is installed in the open space, the substrate being bonded to a lid so that the semiconductor device is isolated from an ambient atmosphere, and the heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the fourteenth aspect of the invention, there is provided a semiconductor equipment comprising a semiconductor device mounted on a heat-radiating board, a ceramic multilayer substrate having a terminal for connecting external wiring and a concave portion for housing the semiconductor device in the middle of the substrate, and a metal wire for electrically connecting the semiconductor device to the terminal of the substrate, both of the heat-radiating board and the substrate being bonded to each other so that the semiconductor device is installed in the concave portion of the substrate, the substrate being bonded to a lid so that the semiconductor device is isolated from an ambient atmosphere, and the heat-radiating board is the same as recited in the ninth aspect of the invention.
According to the fifteen aspect of the invention, there is provided a semiconductor equipment comprising a semiconductor device bonded to a heat-radiating board with a heat-conducting resin, a lead frame bonded to a ceramic insulating substrate, a TAB for electrically connecting the semiconductor device to the lead frame, both of the heat-radiating board and the insulating substrate being bonded to each other so that the semiconductor device is isolated from an ambient atmosphere, and an elastic body of heat-conducting resin interposed between the semiconductor device and the insulating substrate, the heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the sixteenth aspect of the invention, there is provided a semiconductor equipment comprising a semiconductor device bonded onto a first heat-radiating board by use of a metal, a second heat-radiating board to which an earthing board is bonded, the first heat-radiating board being mounted on the earthing board, and a TAB electrically connected to a terminal of the semiconductor device, the semiconductor device being encapsulated by resin, the heat-radiating board being the same as recited in the ninth aspect of the invention.
According to the seventeenth aspect of the invention, there is provided a dielectric board for electrostatic adsorption devices which board is made of the composite material recited above.
According to the eighteenth aspect of the invention, there is provided an electrostatic adsorption device in which, by applying a voltage to an electrode layer, an electrostatic attractive force is generated between a dielectric board bonded to the electrode layer and a body to thereby fix the body to the surface of the dielectric board, the dielectric board being the same as the dielectric board recited in the seventeenth aspect of the invention.
In a composite material related to the invention, Au, Ag, Cu and Al with high electrical conductivity are used as metals and, particularly, Cu is the best because of its high melting point and high strength. As an inorganic compound of the composite material, it is undesirable to use conventional compounds with hardness very different from that of a base metal, such as SiC and Al2O3, as mentioned above. It is desirable to use a compound having a granular shape, relatively low hardness and an average coefficient of linear expansion in a temperature range from room temperature to 300xc2x0 C. of not more than 10xc3x9710xe2x88x926/xc2x0 C. and, more preferably, not more than 7xc3x9710xe2x88x926/xc2x0 C. Copper oxide, tin oxide, lead oxide and nickel oxide are available as such inorganic compounds. Particularly, copper oxide with good ductility is preferred because of its high plastic workability.
A method of manufacturing a composite material related to the invention comprises the steps of melting and casting a raw material comprising copper and copper oxide, performing heat treatment at 800xc2x0 C. to 1050xc2x0 C., and performing cold or hot plastic working.
Further, a method of manufacturing a composite material related to the invention comprises the steps of melting and casting a raw material comprising copper or copper and copper oxide under a partial pressure of oxygen of 10xe2x88x922 Pa to 10""Pa, performing heat treatment at 800xc2x0 C. to 1050xc2x0 C., and performing cold or hot plastic working.
Either cuprous oxide (Cu2O) or cupric oxide (CuO) may be used as the raw material. The partial pressure of oxygen during melting and casting is preferably 10xe2x88x922 Pa to 103 Pa and is more preferably 10xe2x88x921 Pa to 102 Pa. Further, by changing the mixture ratio of the raw material, partial pressure of oxygen, and cooling rate during solidification, etc., it is possible to control the ratio of the Cu phase to the Cu2O phase and the size and shape of the Cu2O phase of the composite material. The proportion of the Cu2O phase is preferably in the range of 10 to 55 vol. %. Especially when the Cu2O phase becomes more than 55 vol. %, the thermal conductivity decreases and the variation of the properties of a composite material occurs, making it inappropriate to use the composite material in a heat-radiating board of a semiconductor equipment. Regarding the shape of the Cu2O phase, the shape of a dendrite formed during solidification is preferred. This is because in the dendrite branches are intricate in a complicated manner, with the result that the expansion of the Cu phase having large thermal expansion is pinned by the Cu2O phase having small thermal expansion. The branches of the dendrite formed during solidification can be controlled, by changing the blending ratio of the raw material or partial pressure of oxygen, to have a Cu phase, to have a Cu2O phase, or to have a CuO phase. Also, it is possible to increase strength by dispersing the granular, fine Cu2O phase in the Cu phase with the aid of a eutectic reaction. The size and shape of the Cu2O phase can be controlled by performing heat treatment at 800xc2x0 C. to 1050xc2x0 C. after casting. Furthermore, it is also possible to transform CuO (which had been formed during solidification) into Cu2O by use of internal oxidation process in the above heat treatment. In other words, this operation is based on the fact that, when CuO coexists with Cu, the transformation of CuO into Cu2O by the following formula (1) is thermally more stable at high temperatures:
2Cu+CuOxe2x86x92Cu+Cu2Oxe2x80x83xe2x80x83(1)
A predetermined period of time is required in order that Formula (1) reaches equilibrium. For example, when the heat treatment temperature is 900xc2x0 C., about 3 hours are sufficient. The size and shape of the fine Cu2O phase formed in the Cu phase by a eutectic reaction can be controlled by the heat treatment.
Regarding a method of melting, in addition to ordinary casting, a unidirectional casting process, a thin-sheet continuous casting process and etc. may be used. In the ordinary casting, dendrites are isotropically formed and, therefore, the composite material is made isotropic. In the unidirectional casting process, the Cu phase and Cu2O phase are oriented in one direction and, therefore, anisotropy can be imparted to the composite material. In the thin-sheet continuous casting process, dendrites become fine because of a high solidification rate and, therefore, dendrites are oriented in the sheet-thickness direction. For this reason, anisotropy can be imparted to the composite material of sheet and, at the same time, it is possible to reduce the manufacturing cost.
Further, in a composite material of the invention, since the Cu phase and Cu2O phase constituting the composite material are low in hardness and have sufficient ductility, cold or hot working, such as rolling and forging, is possible and is performed as required after casting or heat treatment. By working the composite material, anisotropy occurs therein and besides its strength can be increased. Particularly when cold or hot working is performed, the Cu2O phase is elongated and oriented in the working direction and anisotropy in the thermal and mechanical properties occurs in the direction at right angles to the elongated direction. At this time, the thermal conductivity in the elongated and oriented direction is higher than the thermal conductivity at right angles to the oriented direction, and this difference becomes 5 to 100 W/mxc2x7k.